PASI (psoriasis area and severity index)

Psoriasis areas and severity index (PASI) score denotes an objective method of scoring severity of psoriasis, reflecting not only the body surface area but also erythema, induration and scaling

• PASI combines the assessment of the severity of lesions and the area affected into a single score in the range 0 (no disease) to 72 (maximal disease)
• body is divided into four sections based on the estimated area of skin affected:
  
  
  • head - 10%
  • arms - 20%
  • trunk - 30%
  • legs - 40%
    
    
    • Each of these areas is scored by itself, and then the four scores are combined into the final PASI
      • For each section, the percent of area of skin involved, is estimated and then transformed into a grade from 0 to 6:
        
        
        • grade 0: 0% of involved area
          
          
        • grade 1: < 10% of involved area
          
          
        • grade 2: 10-29% of involved area
          
          
        • grade 3: 30-49% of involved area
          
          
        • grade 4: 50-69% of involved area
          
          
        • grade 5: 70-89% of involved area
          
          
        • grade 6: 90-100% of involved area

Within each area, the severity is estimated by three clinical signs:

• erythema (redness)
• induration (thickness)
• desquamation (scaling)
• severity parameters are measured on a scale of 0 to 4

The sum of all three severity parameters is then calculated for each section of skin, multiplied by the area score for that area and multiplied by weight of respective section (0.1 for head, 0.2 for arms, 0.3 for body and 0.4 for legs).

• click for an online PASI calculator
  
  
• click here for more PASI calculation description with guide and images how to classify the severity of redness, thickness and scaling of psoriasis

Be aware that (1):

• PASI and body surface area are not validated for use in children and young people
• erythema may be underestimated in people with darker skin types, such as skin types V and VI on the Fitzpatrick scale

Reference:

1. Psoriasis: The assessment and management of psoriasis. NICE Clinical Guideline (October 2012 - last updated September 2017)